The invention relates to a decoding device for a record carrier having a multi-track digital data pattern includes error protection data wherein each data word comprises not only user symbols but also redundancy symbols. Successive data words are stored on the record carrier in a distributed manner in order to allow for correcting burst errors, there also being provided a detection mechanism for supplying pointer signals for suspect symbols. The decoding device comprising a memory for collecting all symbols of a data word and an error correction element which is fed by the memory in order to perform, if necessary and possible, up to a predetermined maximum number of corrections within a word while using the pointer signals, and to supply an excess signal in the case of an excessive number of errors, and also comprises a selectively activatable concealment element which is fed by the correction element in order to deactivate, under the control of the excess signal, a received, presumably irrepairable user symbol on a user output. A known record carrier of this type is the socalled "Compact Disc" for the storage of high-quality audio, described in detail in Philips Technical Review, Vol. 40 , No. 6 (1982), pp. 151-156 (system aspects) and 166-173 (error protection, and error masking/concealment). "Multi-track" is to be understood to mean herein a carrier for which there is provided a read element which can be displaced between two or more read positions by by displacement transversely of a relative direction of movement between the read element and the carrier, without reversal of the latter movement direction being necessary. For example, a disc can comprise a spiral track as well as a number of concentric tracks. A tape, for example a magnetic tape, may also comprise a plurality of parallel tracks. The same holds true for a drum. A further possibility is the case where successive tracks are diagonally adjacent on a track, such as is used in R-DAT recording (digital audio tape with a rotating head).
Error protection is usually realized on a word basis. Such a word consists of a suitably chosen number of symbols each comprising a uniform number of bits. Because a sub-set of the symbols is redundant, a code having symbol-correcting properties can be implemented. Such a code is usually systematic at the symbol level, but that is not a requirement. The use of record carriers often involves the occurrence of so-called burst errors, which are due to the properties of the record carrier itself (material defects, scratches) as well as to faults during reading. Consequently, a series of successively read symbols may be unreliable: the error density can then easily exceed the correction capability of the code. In order to improve this situation, according to the cited publication the symbols which will ultimately form part of a single code word are stored on the record carrier in a distributed manner. After reading, the distribution is cancelled before correction is initiated. As a result, the symbols of a burst which is not excessively long will be distributed among a number of code words in such way that the error is eventually correctible.
For many symbol-correcting codes the number of symbols to be corrected per code word (t) at the most equals half the number of redundant symbols. In particular cases, this number is found to be too low.
European Patent Application 84913, which corresponds to U.S. Pat. No. 4,477,903, discloses a detection mechanism which forms pointer signals for suspect symbols. When use is made of such erase symbols indicated as being unreliable, the number of erase symbols e to be corrected per code word at the most equals the number of redundant symbols. If the latter number is, for example 4, the following situations may occur; each line indicating a case that can be converted to a correct code word.
t=e=0 PA1 t=0, e=1 PA1 t=0, e=2 PA1 t=0, e=3 PA1 t=0, e=4 PA1 t=1, e=0 PA1 t=1, e=1 PA1 t=1, e=2 PA1 t=2, e=0. PA1 a. normal error symbols that had not been signalled as suspect; PA1 b. symbols that are accompanied by a signalization due to track loss; PA1 c. symbols that are signalled as suspect by an earlier stage of the processing, such as the demodulation step.
Notably if the number of erase symbols amounts to 3 or 4, additional correction will not be possible. If the number of erase symbols amounts to 4, it is even impossible to detect whether the correction has been correctly executed or not. In some cases this detection will be omitted for e=3, but even if that is not the case, a further error will be overlooked in approximately 1/2% (=1/256) of the cases for an 8-bit symbol. Such an overlook can also occur with a different probability in other cases .